1. Field of the Invention
The invention relates generally to multi-photon imaging systems.
2. Discussion of the Related Art
In multi-photon imaging, molecules are excited via absorptions of two or more photons and imaged via light emitted during molecular de-excitations. The imaging light has a different wavelength than the exciting light and is emitted by selected molecular species, e.g., dye molecules. For these reasons, multi-photon imaging can produce tissue-selective images, e.g., of blood or neural tissue. In medical diagnostics, tissue-selective images are useful for in vivo analysis. Unfortunately, rates for multi-photon absorptions are low unless illumination has a high peak intensity. For example, rates for two-photon absorption events grow as a square of the illumination intensity.
For imaging applications, acceptable multi-photon absorption rates usually require intense illumination. Some imaging systems use very short optical pulses to produce the required intense illumination. One such imaging system is a scanning endoscopic microscope 10 illustrated in FIGS. 1 and 2.
Referring to FIG. 1, the scanning endoscopic microscope 10 includes a laser 12, a pre-compensator 14, a transmission optical fiber 16, a remote endoscopic probe 18, and a processor 20. The laser 12 emits short optical pulses with high peak intensities. The peak intensities are high enough to generate acceptable rates of molecular multi-photon absorptions in sample 22. The pre-compensator 14 chirps optical pulses to pre-compensate for the subsequent effects of the chromatic dispersion in the transmission optical fiber 16. The transmission optical fiber 16 delivers the optical pulses to the remote endoscopic probe 18. The remote endoscopic probe 18 scans the sample 22 with the optical pulses and measures intensities of light emitted by molecules that undergo absorptions of two or more photons during the scanning. The processor 20 constructs an image of the sample 22 from measured intensities received via line 24 and electrical data indicative received via line 26. The electrical data is indicative of scanning positions in the sample 22.
Referring to FIG. 2, the probe 18 includes a mechanical oscillator 28, a segment of optical fiber 30, a lens system 32, and a light detector 34. The mechanical oscillator 28 drives the segment of optical fiber 30 so that the fiber end 36 performs an oscillatory 2-dimensional motion. The lens 32 focuses light emitted from the fiber end 36, i.e., optical pulses received from transmission fiber 16, to an illumination spot 38 in the sample 22. The illumination spot 38 makes a scanning motion in the sample 22 that corresponds to the oscillatory 2-dimensional motion of the fiber end 36. The light detector 34 measures intensities of light emitted in response to molecular multi-photon absorptions in the sample 22. The processor 22 uses the measured light intensities and electrical data indicative of the position of the fiber end 36 to construct a scanned image of the sample 22.
Molecular multi-photon absorptions have rates that are acceptably high for imaging if illumination optical pulses have high peak-intensities. Unfortunately, transmission through optical fibers often broadens optical pulses thereby lowering peak intensities. In ordinary optical fibers, both chromatic dispersion and nonlinear optical effects such as self-phase modulation can broaden optical pulses.
Referring to FIG. 1, to inhibit broadening by chromatic dispersion, pre-compensator 14 pre-chirps illumination optical pulses prior to their insertion into transmission optical fiber 16. The chirping places longer wavelength components behind shorter wavelength components in the optical pulses. During propagation through the transmission optical fiber 16, the longer wavelength components propagate faster than the shorter wavelength due to chromatic dispersion. The faster propagation of the longer wavelength components produces temporal narrowing of the pre-chirped optical pulses thereby counteracting the broadening effect chromatic dispersion would otherwise produce in the absence of chirping.
Referring to FIG. 1, to inhibit broadening of pre-chirped optical pulses by nonlinear optical effects, imaging system 10 maintains light intensities in transmission optical fiber 16 at low values. This reduces nonlinear optical interactions, because such interactions have low rates at low light intensities. The light intensities may be maintained at the low values by lowering initial peak intensities of the optical pulses produced by laser 12. The light intensities may also be maintained at low values by using a multi-mode fiber for transmission optical fiber 16. In the multi-mode fiber, light intensities are lower than in a single mode fiber (SMF) especially when a device inserts the optical pulses in a manner that causes the opiical pulses to laterally spread out thereby filling the larger core of multi-mode optical fiber. Unfortunately, a multi-modal fiber can also introduce pulse broadening due to modal dispersion.
It is desirable to have improved systems for producing multi-photon images in-vivo.